Would it work if you made the tile a statue type, then had an inherent flag and a placed flag on the same tile? Shooters actually don't count as beatable enemies no matter what you check in the editor, so as long as they're before the Aquamentus on the enemy list, they should always return no matter what.
And, of course, you wouldn't need to manually place the enemy for the statue. Just use the screen flag. Also ... is this a dungeon boss? If so, you can use "enemies always return" in conjunction with the dungeon boss screen flag, and the battle would reset every time you exited the room until you killed both of the Aquamentus, (the only beatable enemies).
I'm not positive that will all work for you, but outside of a script those seem like the best options to me.
Edited by Aevin, 30 January 2014 - 04:40 PM.